Showing papers on "Powder metallurgy published in 1999"

Full sintering of powdered-metal bodies in a microwave field

Abstract: The use of microwaves to process absorbing materials was studied intensively in the 1970s and 1980s, and has now been applied to a wide variety of materials1,2,3,4. Initially, success in microwave heating and sintering was confined mainly to oxide and some non-oxide ceramics5,6,7,8,9,10,11; but recently the technique has been extended to carbide semimetals12,13,14 used in cutting tools. Here we describe the microwave sintering of powdered metals to full density. We are able to sinter a wide range of standard powdered metals from commercial sources using a 2.45-GHz microwave field, yielding dense products with better mechanical properties than those obtained by conventional heating. These findings are surprising in view of the reflectivity of bulk metals at microwave frequencies. The ability to sinter metals with microwaves should assist in the preparation of high-performance metal parts needed in many industries, for example, in the automotive industry.

Surface oxide and the role of magnesium during the sintering of aluminum

Abstract: The effect of trace additions of magnesium on the sintering of aluminum and its alloys is examined. Magnesium, especially at low concentrations, has a disproportionate effect on sintering because it disrupts the passivating Al2O3 layer through the formation of a spinel phase. Magnesium penetrates the sintering compact by solid-state diffusion, and the oxide is reduced at the metal-oxide interface. This facilitates solid-state sintering, as well as wetting of the underlying metal by sintering liquids, when these are present. The optimum magnesium concentration is approximately 0.1 to 1.0 wt pct, but this is dependent on the volume of oxide and, hence, the particle size, as well as the sintering conditions. Small particle-size fractions require proportionally more magnesium than large-size fractions do.

Phase transformation and thermal expansion of Cu/ZrW2O8 metal matrix composites

Abstract: Powder metallurgy was used to fabricate fully dense, unreacted composites consisting of a copper matrix containing 50–60 vol% ZrW2O8 particles with negative thermal expansion. Upon cycling between 25 and 300 °C, the composites showed coefficients of thermal expansion varying rapidly with temperature and significantly larger than predicted from theory. The anomalously large expansion on heating and contraction on cooling are attributed to the volume change associated with the allotropic transformation of ZrW2O8 between its high-pressure γ-phase and its low-pressure α- or β-phases. Based on calorimetry and diffraction experiments and on simple stress estimations, this allotropic transformation is shown to result from the hydrostatic thermal stresses in the particles due to the thermal expansion mismatch between matrix and reinforcement.

Hydroxyapatite–Ti functionally graded biomaterial fabricated by powder metallurgy

Abstract: A functionally-graded biomaterial (FGM) in a hydroxyapatite (HA)–Ti system was developed by an optimized powder metallurgical process and investigated by microstructural analysis and mechanical tests. The constituents in sintered HA–Ti FGM distribute gradually with the variation in chemical composition, eliminating the macroscopic interfaces, such as that in HA–Ti direct joint. Partial decomposition of HA phase was found in interlayers of the FGM due to the co-existence of Ti during sintering. Mechanical properties in HA–Ti FGM also exhibit gradient distributions. Vickers hardness and Young’s modulus are strongly affected by the porosity. However, bending strength and fracture toughness increase remarkably with the rise of Ti content, especially in the Ti-rich region. Maximum strength and toughness (971.96 Mpa and 29.691 MPam 1/2 respectively) were reached in the pure Ti layer, which are far higher than those of human bone. In addition, the strengthening and toughening mechanism was discussed. In summary, HA–Ti FGM is a promising biomaterial for use as a hard-tissue replacement implant, considering its mechanical behaviors.

Finite element analysis of melting and solidifying processes in laser rapid prototyping of metallic powders

Abstract: To clarify the forming mechanism in laser rapid prototyping, the melting and solidifying processes of metallic powders are simulated by the finite element method, and the calculated amount of the solidified mass is compared with the experimental one. In the simulation, the melted part of the powders is assumed to change into a sphere due to the surface tension and a new finite element mesh is generated in the subsequent analysis. Latent heat and shrinkage due to solidification are taken into account. The thermal conductivity of the powder is measured for various densities and used in the calculation. Temperature distribution within the powders during heating and cooling is calculated to obtain the amount of the melted and solidified part of the powders. Experiments are made with Cu powders using a pulsed Nd:YAG laser. It is found that the amount of the solidified part after a pulse of laser irradiation is affected by the peak power of the laser rather than the duration of irradiation. There is an appropriate peak power of the laser in rapid prototyping of metallic powders. The calculated weights of the solidified powders by several pulses of the laser beam agree well with the experimental ones.

Ultra Grain Refining and Decomposition of Oxide during Super-heavy Deformation in Oxide Dispersion Ferritic Stainless Steel Powder

Abstract: Mechanical milling using a high energy planetary ball mill was applied to the powder mixtures of iron, chromium and yttria (Y2O3) (Fe-24mass%Cr-0-15mass%Y2O3) to introduce a very large strain into the iron-base matrix, and microstructural changes during mechanical milling were investigated in relation to decomposition behavior of Y2O3 particles Mechanical milling of more than 36 ks was long enough to allow the mechanical alloying of iron and chromium powders After the milling of 36 ks, ultrafine bcc crystalline grains of 10 to 20 nm were formed within Fe-24mass%Cr-15 mass%Y2O3 powder mixture and 15 mass% of Y2O3 particles were almost decomposed The resultant powder mixture markedly hardened to about 1000 Hv The decomposition of Y2O3 particles can be explained as being due to the formation of an amorphous grain boundary layer where yttrium and oxygen atoms are enriched As a result, it is proposed that, for the dissolution of Y2O3, bcc crystalline grains should be refined to a nanometric size to provide a sufficient volume fraction of the grain boundary layer, and that Y2O3 particles should be crushed to several nanometers to produce the driving force for the decomposition of Y2O3 particles

Microstructuring by selective laser sintering of metallic powder

Abstract: This paper describes recent work carried out on three-dimensional microstructuring by beam technology — both one-component solid-state laser sintering and two-metal liquid-phase selective laser sintering of metallic powders with high and low melting points. The results show that, due to the surface contact only, the feature size obtained with one-component solid-state sintering is smaller than that with two-metal liquid-phase sintering. The influence of the processing conditions on the type of phases and microstructure evolution are considered. Attempts were also made to create fine structures with the composite powder materials. A few brief examples are demonstrated.

Sintering of nanocrystalline powders

Abstract: The use nanocrystalline materials for novel applications requires the development of cost effective consolidation methods to generate engineering components. Full consolidation of nanoparticles is a sensitive compromise between initial nanopowder characteristics, the several processing conditions in sintering such as pressure, temperature and time, and microstructural changes such as pore elimination and grain growth. This paper reviews lesses in nanopowder densification and discusses which affer commercial potential for the commercial production of components with nanosize grains.

Microwave Combustion Synthesis and Sintering of Intermetallics and Alloys

Abstract: The high melting points and high-temperature stability of many intermetallics=alloys make them useful for high-temperature applications, especially for corrosion and oxidation resistance. Some intermetallics ®nd applications in wear components and oxidation barriers as well as reinforcing phases in many metallurgical systems, including superalloys. Intermetallics and alloys have been prepared by a variety of methods including casting, mechanical alloying, gas atomization, sintering, combustion synthesis, etc. In the combustion synthesis process (also called self-propagating high-temperature synthesis), two or several mixed reactants react exothermically in a self-sustaining manner, to form products due to the large difference in free energy and enthalpy between the product and reactants [1, 2]. In the conventional combustion process, reactants are ignited in a resistance furnace or by a hot ®lament or laser. The novelty of the present method is the use of microwave energy to initiate the combustion reaction between metals to form intermetallic compounds or alloys. Investigations in the area of microwave combustion synthesis previously were successful only for ceramics and composites [3]. A second novelty of this method is the sintering of metal±metal systems using microwaves. Microwave sintering of white ceramics with marginal success has been studied for two decades (see reviews in [4, 5]). Recently, in our laboratories, it was used in a new con®guration for sintering to essentially theoretical density translucency and even transparency of a wide range of ceramics: alumina, mullite, spinel, etc. [6, 7]. The earlier disputed question ` Is there microwave effect?'' has been de®nitively answered not only in the radically enhanced (one to two orders of magnitude) kinetics of sintering but the totally different reaction pathways for reactions such as BaCO3 ‡ TiO2 ! BaTiO3 ‡ CO2. So far, there have been no reports of metal±metal chemical reaction processes or sintering through solid-state or liquid-state diffusion using microwave energy, possibly because metals invariably cause plasma discharges within a microwave cavity and are therefore unsuitable for microwave synthesis and sintering. It was recently demonstrated in this laboratory [8], however, that a wide variety of compacted metal powders and ordinary commercial powder metal compacts can couple effectively with microwave ®elds and be synthesized or sintered at least as well as or better than conventional heating. This letter reports on the use of this new method for the preparation of intermetallics and alloys by combustion synthesis and sintering using microwave energy. Microwave heating is fundamentally different from conventional heating because all the energy is directed at the workpiece, as is also the case in laser processing. In microwave processes, the heat is generated internally through material±microwave interaction instead of originating from an externalheating source. This internal volumetric heating results in an energy diffusion that is reverse to the direction of those observed in conventional heating. As noted, the large body of work existing on microwave processing of polymer and ceramic materials has been summarized elsewhere [4, 5] but will be repeated here. For metal processing, as expected, there is almost no work. Even ®nely powdered metals do not couple well with microwave energy at room temperature. Walkievwicz et al. [9] reported that 25 g samples of powder of a half-dozen metals (Al, Co, Fe, etc.) exposed for 1 h in a 2.45 GHz cavity reached temperatures of 120±770 8C and showed no sintering. One US patent [10] refers to composites of metals and oxides being sintered in microwaves. We have demonstrated that microwave coupling ef®ciency of metals is greatly increased by increasing the temperature of the sample. This can be achieved by starting with a hybrid method in which an external susceptor is used to trigger the reaction, but we established that exactly the same result can also be achieved in a pure microwave system but with slower heating rates. Once the material is heated to a ` critical'' temperature, enhanced microwave absorption becomes suf®cient to cause self-heating, and the susceptors can be removed. As the temperature of material increases, the absorption of microwave energy by the material also increases. Extensive work on commercial powder metal samples [8] has established the experimental feasibility of sintering virtually all powder metal compacts. Our focus was on intermetallic synthesis and sintering. Stoichiometric and nonstoichiometric mixtures of metal powders in systems Ti±Al, Ni±Al, Fe±Al, Nb±Al, Ta±Al, Ge±Al, Cu±Al, Co±Al, Mo±Al, W±Al, Pt±Al, Pd±Al, Sb±Al, Zr±Al, Cr± Al, Nb±Ge, Ti±Ni, Ti±Co, Ti±Fe, Cu±Ti, Cu±Zn, Fe±Ni, Ni±Mg, Ni±Zr, Ti±Cu±Al, Cu±Zn±Al, Nb±Ti±Al, Nb±V±Al, Nb±Zr±Al, Ta±Al±Fe, Ni±

Review positron annihilation in fine-grained materials and fine powders - an application to the sintering of metal powders

Abstract: We consider the specific problem of the influence of an inhomogeneous distribution of defects in solids on positron annihilation characteristics. In detail, we investigate the effect of micro-structure, i.e. dislocations, vacancies, vacancy clusters, grain and subgrain boundaries, pores or inner surfaces, on positron lifetime spectroscopy. Only few materials show such small grain sizes that positron annihilation is affected. One example are powder compacts, made out of small and fine-grained powder, during different stages of the sintering process. All samples generically show positron trapping at grain boundaries (τGB ≈ 300 ps) and at surfaces (τsurf = 500–600 ps). τGB = 300 ps corresponds to small voids consisting of roughly eight vacancies. Different defects can lead to similar annihilation parameters. Hence, we compare the lifetime data obtained from porous and fine-grained samples to the kinetics of defect annealing after irradiation and plastic deformation, e.g. the thermal stability of dislocations or vacancy clusters. We conclude that τGB ≈ 300 ps is apparently not related to vacancy clusters in the matrix, but is due to positron trapping at large-angle grain boundaries. This large open volume is in contrast to common grain boundary models. The change of porosity and grain size with temperature, i.e. during sintering, has been determined in a correlated study by metallography and X-ray line-profile analysis. The effective powder particle size ranges from ≈0.5 to ≈15 μm, while the grain sizes are always smaller. The only detectable lattice defects in all samples above recrystallization temperature are grain boundaries, besides a surface component in very fine powders.

Functionally graded metal substrates and process for making same

Abstract: The invention provides for a functionally-graded metal substrate that is made of at least two metal compositions, a functional insert and a surrounding body that surrounds the functional insert. In a preferred embodiment of the invention a functional insert powder composition of loose powder metal is place in a compact of a surrounding body powder composition and both metal compositions are sintered in a sintering furnace to form a sintered part. The sintered part is infiltrated in part or in whole with a molten metal compound to produce a functionally graded metal substrate having a density of at least 90 % of theoretical. A heat-generating component such as a chip can be attached to the metal substrate for use in microelectronic packaging. When the functionally-graded metal substrate has two discrete compositions of copper/tungsten the surrounding body which has a CTE that ranges from about 5.6 ppm/C to about 7 ppm/C constrains the expansion of the functional insert which has a thermal conductivity that ranges from about 200 W/mK to about 400 W/mK.

Powder and binder systems for use in metal and ceramic powder injection molding

Abstract: A powder injection molding composition or feedstock is made of 70% or more by weight of a powdered metal or ceramic and 30% or less by weight of a binder system. The binder system contains a sufficient amount of polypropylene or polyethylene to hold the so-called brown preform of the molded metal or ceramic powder together for the sintering step of the injection molding process and a sufficient amount of partially hydrolyzed cold water soluble polyvinyl alcohol water and plasticizer to facilitate molding of the composition into the so- called green preform of the article to be manufactured. The debinding step of the injection molding process for transforming the green preform into the brown preform consists simply of immersing the green preform in water at ambient temperature to dissolve the polyvinyl alcohol. The binder system is nonhazardous, safe, harmless and fully degradable.

The preparation and the properties of microcrystalline and nanocrystalline CuCr contact materials

Abstract: The microcrystalline and nanocrystalline CuCr alloys prepared by high-energy ball milling and hot pressing were investigated in this paper. The experimental results show that the nanocrystalline Cu-Cr alloy powders are obtained by high energy ball milling, and the milled powders appear flaked or equiaxed morphology with or without liquid medium addition. The grain size of near fully dense alloys consolidated at 850 and 1200 K from milled powders is less than 100 nm and about 2-3 /spl mu/m, respectively. The ability to withstand high voltage of the nanocrystalline CuCr materials in vacuum is much higher than that of microcrystalline materials. The breakdown first takes place on the Cu-rich phase in the microcrystalline CuCr materials. For nanocrystalline CuCr materials, the breakdown exhibits a diffusional feature, in which the arc can move to the whole contact surface in a breakdown.

Surface oxide debonding in field assisted powder sintering

Abstract: Field activated sintering techniques (FAST) have been applied to two high-temperature powder materials: tungsten and NiAl. High and atomic resolution electron microscopy (HREM/ARM) of tungsten powder sintered via FAST showed essentially clean boundaries. Analytical transmission electron microscopy (TEM) of FAST sintered NiAl also showed boundaries free of surface oxide layer(s). However, small alumina precipitates were found at and near prior particle (grain) boundaries. The boundary cleaning and precipitation phenomena are manifestations of an applied pulsed electric field.

On the use of trace additions of Sn to enhance sintered 2xxx series Al powder alloys

Abstract: The mechanical properties of a typical sintered aluminium alloy (Al-4.4Cu-0.8Si-0.5Mg) have been improved by the simultaneous use of trace additions of Sn, high sintering temperatures and modified heat treatments. Tin increases densification, but the Sn concentration is limited to less than or equal to 0.1wt% because incipient melting occurs during solution treatment at higher Sn levels. A sintering temperature of 620 degrees C increases the liquid volume over that formed at the conventional 590 degrees C sintering temperature. However, the higher sintering temperature results in the formation of an embrittling phase which can be eliminated if solution treatment is incorporated into the sintering cycle (a modified TS heat treatment). These conditions produce a tensile strength of 375 MPa, an increase of nearly 20% over the unmodified alloy. (C) 1999 Elsevier Science S.A. All rights reserved.